While the number and unit are explicitly represented when a quantity is written, the uncertainty is an aspect of the measurement result that is more implicitly represented and will be discussed later. The number in the measurement can be represented in different ways, including decimal form and scientific notation. Scientific notation is also known as exponential notation; a review of this topic can be found in Essential Mathematics.
For example, the maximum takeoff weight of a Boeing ER airliner is , kilograms, which can also be written as 2. The mass of the average mosquito is about 0. Units , such as liters, pounds, and centimeters, are standards of comparison for measurements.
When we buy a 2-liter bottle of a soft drink, we expect that the volume of the drink was measured, so it is two times larger than the volume that everyone agrees to be 1 liter. The meat used to prepare a 0. Without units, a number can be meaningless, confusing, or possibly life threatening. Not only will this be confusing to the medical professional giving the dose, but the consequences can be dire: mg given three times per day can be effective as an anticonvulsant, but a single dose of g is more than 10 times the lethal amount.
We usually report the results of scientific measurements in SI units, an updated version of the metric system, using the units listed in Table 1.
Other units can be derived from these base units. Sometimes we use units that are fractions or multiples of a base unit. Ice cream is sold in quarts a familiar, non-SI base unit , pints 0. We also use fractions or multiples of units in the SI system, but these fractions or multiples are always powers of Fractional or multiple SI units are named using a prefix and the name of the base unit.
The prefixes used and the powers to which 10 are raised are listed in Table 2. The initial units of the metric system, which eventually evolved into the SI system, were established in France during the French Revolution. The original standards for the meter and the kilogram were adopted there in and eventually by other countries. This section introduces four of the SI base units commonly used in chemistry. Other SI units will be introduced in subsequent chapters. The standard unit of length in both the SI and original metric systems is the meter m.
A meter is about 3 inches longer than a yard Figure 1 ; one meter is about Figure 1. The relative lengths of 1 m, 1 yd, 1 cm, and 1 in. Figure 2. The standard unit of mass in the SI system is the kilogram kg. A kilogram was originally defined as the mass of a liter of water a cube of water with an edge length of exactly 0. It is now defined by a certain cylinder of platinum-iridium alloy, which is kept in France Figure 2. Any object with the same mass as this cylinder is said to have a mass of 1 kilogram.
One kilogram is about 2. Temperature is an intensive property. The SI unit of temperature is the kelvin K. Celsius degrees are the same magnitude as those of kelvin, but the two scales place their zeros in different places. Water freezes at The conversion between these two units and the Fahrenheit scale will be discussed later in this chapter.
The SI base unit of time is the second s. Alternatively, hours, days, and years can be used. We can derive many units from the seven SI base units. Fractional or multiple SI units are named using a prefix and the name of the base unit. The prefixes used and the powers to which 10 are raised are listed in Table 1.
Need a refresher or more practice with scientific notation? Visit this site to go over the basics of scientific notation. The initial units of the metric system, which eventually evolved into the SI system, were established in France during the French Revolution. The original standards for the meter and the kilogram were adopted there in and eventually by other countries. This section introduces four of the SI base units commonly used in chemistry.
Other SI units will be introduced in subsequent chapters. The standard unit of length in both the SI and original metric systems is the meter m. A meter is about 3 inches longer than a yard Figure 1. The standard unit of mass in the SI system is the kilogram kg. This object was originally one liter of pure water, and more recently it was a metal cylinder made from a platinum-iridium alloy with a height and diameter of 39 mm Figure 1.
In May , this definition was changed to one that is based instead on precisely measured values of several fundamental physical constants. One kilogram is about 2. Temperature is an intensive property. The SI unit of temperature is the kelvin K. Celsius degrees are the same magnitude as those of kelvin, but the two scales place their zeros in different places.
Water freezes at The conversion between these two units and the Fahrenheit scale will be discussed later in this chapter. The SI base unit of time is the second s. Alternatively, hours, days, and years can be used. We can derive many units from the seven SI base units.
For example, we can use the base unit of length to define a unit of volume, and the base units of mass and length to define a unit of density. Volume is the measure of the amount of space occupied by an object. The standard SI unit of volume is defined by the base unit of length Figure 1. The standard volume is a cubic meter m 3 , a cube with an edge length of exactly one meter. To dispense a cubic meter of water, we could build a cubic box with edge lengths of exactly one meter.
This box would hold a cubic meter of water or any other substance. Very small distances can be expressed in units such as the millimeter or the micrometer. In addition to the fundamental units, SI also allows for derived units based on a fundamental unit or units.
There are many derived units used in science. For example, the derived unit for area comes from the idea that area is defined as width times height. This is sometimes spoken as "square meters. Volume is the amount of space occupied by a sample of matter.
The volume of a regular object can be calculated by multiplying its length by its width and its height. Since each of those is a linear measurement, we say that units of volume are derived from units of length. This very large volume is not very convenient for typical use in a chemistry laboratory. Another definition of a liter is one-tenth of a meter cubed.
Because 1 L equals 1, mL, we conclude that 1 mL equals 1 cm 3 ; thus, these units are interchangeable. Units not only are multiplied together but also can be divided. The word per implies division, so velocity is determined by dividing a distance quantity by a time quantity.
Later, we will see other derived units that can be expressed as fractions.
0コメント